In eukaryotes, the cell cycle is a cyclical series of biological events that certain asexual cells go through. In essence, the cell cycle is highlighted by a sequence of events such as the duplication of DNA via DNA replication, in preparation to cell division wherein the parent cell divides and gives rise to two genetically identical daughter cells. The cell cycle is comprised of these fundamental events: (1) resting phase (Gap 0), (2) interphase (Gap 1, S phase, Gap 2), and (3) cell division (i.e. mitotic phase and cytokinesis).

Gap 0 phase is also referred to as the resting phase. It is because it is in this phase when the cell following cell division enters a resting period. The cell is inactive in a way that it exits the cell cycle and is not preparing itself to undergo cell division. It is a way of the cell to preserve important physiologic function for a longer period of time. This phase may be irreversible or reversible depending on the cells involved. For instance, tissue stem are cells (e.g. muscle stem cells, intestinal stem cells, neural stem cells, etc.) remain in Gap 0 phase and be in a quiescent state for an indefinite period until external stimuli activate them into re-entering the remaining phases of the cell cycle all through cell division. Irreversible Gap 0 phase (i.e. terminal Gap 0) is observed among senescent cells, as well as fully differentiated cells such as myocytes, osteocytes, etc.